The present invention, in some embodiments thereof, relates to energy conversion, and more particularly, but not exclusively, to electrochemical cell systems which utilize air and which can be utilized for forming batteries and fuel cells, and to applications of such batteries and fuel cells.
The continuous search for clean, sustainable and cost-effective source of energy has pushed materials and electrochemical sciences to development of new materials and technologies for portable power sources. The prime objectives include reducing the size of the power source and at the same time increasing its energy or power density, along-side of reducing cost and toxic hazards while increasing safety and ease of use thereof.
Electrochemical cell devices, referred to in the art as “batteries”, are composed of one or more electrochemical or voltaic cells, which store chemical energy and provide it as available electricity due to a potential difference between its electrodes. Types of electrochemical cells include galvanic cells, electrolytic cells, fuel cells, flow cells, and voltaic cells, each comprising two half-cells: one for the oxidation reaction of the chemical fuel (negative electrode or anode) and one for the reduction reaction of the oxidant (positive electrode or cathode). Batteries generate electricity without combustion of the fuel and oxidizer. As opposed to what occurs with other methods of electricity generation, the chemical energy is converted into electrical current and some heat, driven by the redox potential difference between the two halves of the cells. Batteries are therefore typically characterized by having a positively charged anode, a negatively charged cathode, an ion-conducting material referred to as an electrolyte, and conducting negative and positive terminals which conduct the resulting electric current in and out of an electric circuit.
A fuel cell (FC) is a particular type of electrochemical cell device (battery) that continuously converts chemical energy directly to electrical energy as long as a fuel (commonly hydrogen, hydrogen-generating compounds or metallic anode material) and an oxidant (commonly oxygen) are supplied. Fuel cells are characterized by high efficiency compared to internal combustion engines. In addition, fuel cells are ecologically friendly and can function under a wide range of physical conditions.
The development of fuel cells is one of the main directions in the field of new power engineering. Several types of fuel cells based on H2/O2, NaBH4/H2O2, phosphoric acid, molten carbonate, direct methanol and solid oxide were developed in the last two decades. However, these fuel cells are still far from mass production due to multiple practical limitations.
Metal-air batteries have been attracting the electrochemistry research and development community for the last fifty years. Their attractiveness lies in the principle that the cathodic reaction is a catalyzed reduction of oxygen consumed from ambient air rather than oxygen stored in the system. The air battery using the oxygen in air as a positive electrode active material does not require any space for incorporating the active material, and is hence expected to have a high capacity. This aspect results in high energy densities (measured in watt-hour, or Whr) and high specific energies (measured in watt-hour per kilogram, or Whr/kg) for the metal-air batteries.
The general structure of a metal-air battery, such as a lithium-air secondary battery, includes a catalyst layer, an air positive electrode (cathode), a negative electrode (anode), a polymer electrolyte film interposed between the anode and cathode, and an oxygen permeation film laminated on the air cathode. The air cathode may contain, for example, a polymer electrolyte film comprising polyacrylonitrile, ethylene carbonate, propylene carbonate, and LiPF6, which is press-bonded to a nickel or aluminum mesh. Alternatively, the cathode is formed of a lithium foil, and this four-layer laminated body is sealed in a laminate bag. The catalyst layer for ionizing the oxygen in the ambient air can be made, for example, of acetylene black (a form of carbon black which is a fine black powder of amorphous carbon obtained by the incomplete combustion of hydrocarbons) containing cobalt.
Most metal-air batteries, including Zn-air, Al-air and Fe-air, utilize aqueous alkaline solutions, mainly potassium hydroxide (KOH), due to the high conductivity of such electrolyte and the superb ability to regulate the reduced oxygen ion into hydroxide anions. Zn-air batteries has received broad attention in the 1960's and 1970's, with the development of commercialized primary cells, in coin type structure, for hearing aids operation, although in the last ten years there has been an enormous effort to construct large scale Zn-air batteries for electric vehicles.
The Al-air couple has higher theoretical densities compared to its competitors (Zn-air and Fe-air) and was under investigation as a suitable power source for vehicles and some stationary applications. However, high open circuit corrosion rates lead to the consumption of the Al metal anode without any usable power output.
The lithium-air battery presents the highest theoretical specific energy value (11,246 Whr/kg). Although theoretically high energy density is expected from a lithium-air cell system, nontrivial challenges associated with a practical lithium-air cell exist. For example, Li suffers from severe corrosion in alkaline electrolytes and safety concerns are still unresolved with aqueous systems. In addition, lithium-air cells are sensitive to anode passivation due to air/moisture contamination, the operating current density is typically very low (less than 100 μA/cm2). In 1996, researchers reported a conducting polymer electrolyte-based secondary lithium-air battery. This battery showed open circuit potential (OCP) of about 3 V and working voltage in the range of 2 to 2.8 V. However, good coulombic efficiency was kept for only a few cycles.
Overall, the majority of these fuel cell technologies suffer from a range of drawbacks, such as size-, weight- and capacity limitations, hazardous and/or toxic components and by-products, and cost effectiveness over the entire life-cycle of the device including environmentally safe disposal. Other obstacles associated with fuel cell development include complex electrode and cell design, catalysts poisoning and mechanical instability, high catalyst cost, low potential and slow oxidation kinetic.
It is known in the art that silica undergoes rapid corrosion when in contact with aqueous alkaline solutions such as a KOH solution [Seidel, H. et a, J. Electrochem. Soc., 137 (1990) 3626, and Glembocki, O. J. et al., J. Electrochem. Soc., 138 (1991) 1055], a trait which has served the semiconductor industry for years for silicon etching. Additional background art includes U.S. Pat. No. 4,943,496 and U.S. Patent Application Nos. 20060255464, 20080096061, 20090208791 and 20090208792.